Fix bug where backslashes in inline diffs could be dropped

Closes #2253

CHANGELOG

@@ -1,6 +1,7 @@
 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.14.0 (unreleased)
+  - Fix bug where backslashes in inline diffs could be dropped (Stan Hu)
   - Disable turbolinks when linking to Bitbucket import status (Stan Hu)
   - Fix broken code import and display error messages if something went wrong with creating project (Stan Hu)
   - Fix corrupted binary files when using API files endpoint (Stan Hu)

lib/gitlab/inline_diff.rb

@@ -46,8 +46,11 @@ module Gitlab
           end
           last_the_same_symbols += 1
           last_token = first_line[last_the_same_symbols..-1]
-          diff_arr[index+1].sub!(/#{Regexp.escape(last_token)}$/, FINISH + last_token)
-          diff_arr[index+2].sub!(/#{Regexp.escape(last_token)}$/, FINISH + last_token)
+          # This is tricky: escape backslashes so that `sub` doesn't interpret them
+          # as backreferences. Regexp.escape does NOT do the right thing.
+          replace_token = FINISH + last_token.gsub(/\\/, '\&\&')
+          diff_arr[index+1].sub!(/#{Regexp.escape(last_token)}$/, replace_token)
+          diff_arr[index+2].sub!(/#{Regexp.escape(last_token)}$/, replace_token)
         end
         diff_arr
       end

spec/lib/gitlab/diff/inline_diff_spec.rb

@@ -0,0 +1,39 @@
+require 'spec_helper'
+
+describe Gitlab::InlineDiff do
+  describe '#processing' do
+    let(:diff) do
+      <<eos
+--- a/test.rb
++++ b/test.rb
+@@ -1,6 +1,6 @@
+ class Test
+   def cleanup_string(input)
+     return nil if input.nil?
+-    input.sub(/[\\r\\n].+/,'').sub(/\\\\[rn].+/, '').strip
++    input.to_s.sub(/[\\r\\n].+/,'').sub(/\\\\[rn].+/, '').strip
+   end
+ end
+eos
+    end
+
+    let(:expected) do
+      ["--- a/test.rb\n",
+       "+++ b/test.rb\n",
+       "@@ -1,6 +1,6 @@\n",
+       " class Test\n",
+       "   def cleanup_string(input)\n",
+       "     return nil if input.nil?\n",
+       "-    input.#!idiff-start!##!idiff-finish!#sub(/[\\r\\n].+/,'').sub(/\\\\[rn].+/, '').strip\n",
+       "+    input.#!idiff-start!#to_s.#!idiff-finish!#sub(/[\\r\\n].+/,'').sub(/\\\\[rn].+/, '').strip\n",
+       "   end\n",
+       " end\n"]
+    end
+
+    let(:subject) { Gitlab::InlineDiff.processing(diff.lines) }
+
+    it 'should retain backslashes' do
+      expect(subject).to eq(expected)
+    end
+  end
+end